Many varied types of non-aqueous rechargeable lithium batteries are used commercially for consumer electronics applications. Typically, these batteries employ a lithium insertion compound as the active cathode material, a lithium containing material of some sort (eg. pure lithium metal, lithium alloy, lithium insertion compound) as the active anode material, and a non-aqueous electrolyte. An insertion compound is a material that can act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms).
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Presently available lithium ion batteries are high voltage systems based on LiCoO.sub.2 cathode and coke or graphite anode electrochemistries. However, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material. These batteries employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
The excellent reversibility of this insertion makes it possible for lithium ion batteries to achieve hundreds of battery cycles. However, a gradual loss of lithium and/or buildup of impedance can still occur upon such extended cycling for various reasons and particularly at higher operating voltages. This in turn typically results in a gradual loss in delivered capacity with cycle number. Researchers in the art have devoted substantial effort to reducing this loss in capacity. For instance, co-pending Canadian patent application serial number 2,150,877, filed Jun. 2, 1995, and titled `Use of P.sub.2 O.sub.3 in Non-aqueous Rechargeable Lithium Batteries` discloses a means for reducing this loss which involves exposing the electrolyte to P.sub.2 O.sub.5. However, P.sub.2 O.sub.5 shows at best only limited solubility in typical non-aqueous electrolytes and can be somewhat awkward to use in practice. Alternatives which are soluble may be more convenient, but it is unclear why such exposure is effective and hence what compounds might serve as effective alternatives.
B.sub.2 O.sub.3 is a common chemical that is extensively used in the glass industry, and its properties are well known. B.sub.2 O.sub.3 has also been used in the lithium battery industry for a variety of reasons. In most cases, the B.sub.2 O.sub.3 is used as a precursor or reactant to prepare some other battery component. However, Japanese published patent application 07-142055 discloses that lithium batteries can show improved stability characteristics to high temperature storage when using lithium transition metal oxide cathodes which contain B.sub.2 O.sub.3. Also, co-pending Canadian patent application serial number 2,175,755, filed May 3, 1996, and titled `Use of B.sub.2 O.sub.3 additive in Non-aqueous Rechargeable Lithium Batteries` discloses that B.sub.2 O.sub.3 additives can be used to reduce the rate of capacity loss with cycling in rechargeable lithium batteries and that this advantage can be obtained by having the additive dissolved in the electrolyte. However, the reason that the additive resulted in an improvement with cycling was not understood. In a like manner, Japanese published patent application 09-139232 also discloses that the use of B.sub.2 O.sub.3, or possibly certain other B containing compounds, can improve the cycling and storage characteristics of lithium rechargeable batteries.
Certain other compounds containing boron, oxygen, carbon, and hydrogen have been used historically in battery and/or fuel cell applications. For instance, trimethyl borate has been used as a precursor in a process to make an electrode substrate (as in Japanese laid-open patent application 07-105955, a precursor B-containing compound was kneaded in with the other electrode components before heat treating the mixture to 1000 degrees C.). Boron-oxygen-carbon-hydrogen containing compounds have also been used in the preparation of lithium haloboracite (a lithium-boron-oxygen-halogen containing material) solid electrolyte films for battery usage (as in Japanese laid-open patent application 06-279195).
Recently, researchers have discovered that certain compounds containing boron, oxygen, carbon, and hydrogen can serve as improved electrolyte additives in rechargeable lithium batteries. For instance, in Canadian patent application serial no. 2,196,493, filed Jan. 31, 1997 by a common applicant and having the same title as the instant application, fade rate reducing additives for rechargeable lithium battery electrolytes are disclosed. The fade rate reducing additives comprised a (BO).sub.3 boroxine ring.
Further, in international patent application WO 97/16862 by C. A. Angell et al., improved electrolytes for lithium rechargeable batteries are disclosed wherein the solvent of the electrolyte consists predominantly of a liquid boron electrolyte solvent. The disclosed boron electrolyte solvents all comprise boron atoms which are bonded to two or three oxygen atoms. The electrolytes showed a wider electrochemical stability window than other conventional electrolytes.
Also, Japanese published patent application number 09-120825 of Sanyo discloses the use of various boronate esters and/or borinate esters in lithium secondary batteries in order to suppress self discharge during storage.
For various historical reasons, BF.sub.3 and complexes containing BF.sub.3 have also been employed in primary or non-rechargeable batteries before. (Herein, the term `complex` is defined as a `complex substance in which the constituents are more intimately associated than in a simple mixture` in accordance with the definition in Webster's Ninth New Collegiate Dictionary, 1984, Merriam-Webster Inc.) In U.S. Pat. No. 3,915,743, Varta discloses a primary battery having a lithium metal anode, a sulfur cathode, and which operates below about 2.5 V. The battery comprised a BF.sub.3 adduct of organic solvents (such as dimethyl carbonate or 1,2 dimethoxy ethane) to prevent the formation of polysulphides.
Further, Japanese published patent application number 02-158059 of Tokai Carbon Co. discloses primary or non-rechargeable batteries comprising an aromatic nitrogen compound dissolved in the electrolyte. In examples in this application, BF.sub.3 was used as an electrolyte additive.
BF.sub.3 has also been employed in the assembly of rechargeable lithium batteries before. In Japanese published patent application number 59-154767, Hitachi Maxell discloses a rechargeable lithium battery containing a Li halide salt and BF.sub.3 wherein the BF.sub.3 reacts with the lithium halide salt to form a product which has advantages over LiBF.sub.4 salt. In this disclosure, residual BF.sub.3 is removed prior to assembling the battery. Thus, unreacted BF.sub.3 does not remain in the electrolyte. The resulting electrolyte is more stable at high temperature. In the disclosure, it was mentioned that the complex DME.BF.sub.3 might be used instead of BF.sub.3.
While the preceding prior art may employ BF.sub.3 and/or complexes containing BF.sub.3 in primary batteries or in the assembly of secondary batteries, it appears that BF.sub.3 and/or complexes containing BF.sub.3 have not been used for purposes of improving the fade rate of rechargeable lithium batteries.